The present invention relates generally to data links for telecommunications transmission facilities and, more particularly, to a phantom (out of band) data link for performance monitoring information that uses existing telecommunications transmission facilities. The invention, in effect, allows a telephone company central office and a remote location to communicate with each other over existing telephone lines, without "interfering" with, or displacing, any telephone communications being sent over the lines. Accordingly, the telephone company central office can better diagnose problems that may occur at the remote locations.
The Bell telephone system in the United States, for example, has widely utilized time multiplexing, pulse code modulation systems. Such systems have been designated as "T carriers." As is well known by those of ordinary skill in the art, the first generation of multiplexers designed to feed the T1 system was the D1 channel bank. The channel banks have evolved through the D5 series. The "D" channel bank commonly provides multiple DS-1 signals that are carried on the T1 systems. Each T1 carries 24 two-way channels on two pairs of 15 exchange grade cables. One pair of cables is provided for each direction of transmission.
The data, or "payload," signals to be sent over the transmission lines are sent differentially on the Tip-Ring pair. A longitudinal, or common mode, direct current is applied to the simplex lead to power T1 line repeaters. "Payload" signals are received by the telephone company central office and are transmitted, via the cables, to a series of regenerative repeaters. Such repeaters are spaced along the cables approximately every 6,000 feet.
The information on such a pulse code modulated system is transmitted in the form of bipolar or alternate mark inversion (AMI) pulses. The first repeater receives the data from the central office repeater, but because of transmission line losses, noise, interference, and distortion, the signal will have degenerated. The repeater recognizes the presence or absence of a pulse at a particular point in time and thereafter, if appropriate, regenerates a clean, new pulse. A regenerative repeater, or "line repeater," is powered by the transmission cable itself to generate the new pulses. The new pulses are transmitted by the line repeater along more cable to either another line repeater or to a Network Interface Unit ("NIU"). The NIU, in turn, transmits signals to equipment on the customer premises.
NIUs commonly have the capability to identify errors in the data received over the cable and responsively provide a signal to the central office that the errors have occurred. Errors that can be detected by the NIU include, for example, errors in signaling, format, bipolar violations, out of frame data, or loss of signal, as well as the disconnection of equipment by the customer.
Thus, for example, a cyclic redundancy check ("CRC") may be employed to determine whether the "remainders" (determined under the CRC method) are consistent. If they are not, the NIU will have detected corrupt data, and the central office may need to be advised of the errors by the NIU.
Unfortunately, many of the presently available apparatus and methods for transmitting perturbance monitoring information from the NIU to the central office require using part of the available bandwidth, which would otherwise be available for carrying payload. Thus, part of the payload between the customer's equipment and the central office is, in effect, displaced by the data from the NIU advising the central office of performance information. Since cable is an expensive capital investment, however, such displacement should be minimized.
In addition, the central office may wish to send a signal to the NIU to inquire about historical performance information. The NIU will then responsively provide the requested data to the central office. Such data requested could include statistics on, for example, the number of seconds of severely corrupted data received by the NIU, the historical number of bipolar violations noted by the NIU, or the time when the last substantial burst of corrupt, or error-ridden, data was received by the NIU. Again, the transmission of a request for information by the central office and the resulting transmission of performance information by the NIU may, in some cases, effectively displace a portion of the cables' payload.
The ability to transmit and receive performance monitoring information between the central office and the NIU in an efficient, nonintrusive, and inexpensive manner is thus important. However, such a system must not substantially interfere with the normal performance of the transmission facility. In addition to thus being out of band, or "phantom," the data link should also not be affected by the passage of low frequency (less than 180 Hz) signals through the network, since these signals might be chosen as a test frequencies over the transmission lines as well as the fact that these frequencies are often used for ringing in the telephone plant. For these reasons, the data link should be robust to these interferences.
Moreover, some "intelligent" line repeaters also include a dead loop feature. In this mode, a break in the transmission line or a disconnection of the customer's equipment from the NIU causes the line repeater or the NIU to "dead loop," such that any signal transmitted from the central office is simply rerouted back to the central office. Accordingly, the central office is advised of the abnormality along the transmission cables. The "dead loop" condition may be released if, for example, the line is corrected or the customer's equipment is reconnected to the NIU. Again, the data link must operate when the lines are restored to a normal, or "cut through," mode of operation.
Furthermore, the NIU may be called upon to power up the customer's channel service units. The dam link must be able to accommodate such requirements of the NIU by the customer's premises equipment.